1. Field of the Invention
The present invention relates to a television signal sub-band coder/decoder with various levels of compatibility. It applies in particular to systems for transmitting and receiving digital video signals.
2. Discussion of the Background
In television image transmission systems the multiplicity of resolution formats employed for conventional interlaced television, videophone, sequential television, and the advent of interlaced or sequential high-resolution television systems make it necessary to formulate compatible coding systems, be they upwardly or downwardly directed resolutions.
In upwards compatibility, a receiver working in a specified video format must be able to receive and display a signal transmitted in accordance with a higher format, that is to say a format according to which the resolution of the signal is greater. This is the case for example in a signal for sequential high-definition television, known by the abbreviation HDTV, which itself possesses a format higher than that of a conventional interlaced television signal. Conversely, in the case of downwards compatibility a receiver must be able to receive and display a signal of lesser resolution than its working format. Naturally, the diversities of resolutions entail a diversity of coding and decoding devices to be implemented and considerably complicate the handling of communications.
A solution to these problems which is known from Patent Application FR 2 654 887 likewise filed in the name of the Applicant, consists in carrying out at the level of the transmission coders a partitioning of each image to be transmitted into sub-bands in accordance with a same tree structure by filtering and decimation of the spatial frequency band of the signals to be transmitted whatever family they belong to, so as independently to code the signals of each sub-band before transmitting them by multiplexing over the transmission channel. On reception the decoders carry out the demultiplexing of the coded signals received in each sub-band by decoding the signals relating to each sub-band as a function of the resolution of the signals transmitted and of the inherent insolution of the receivers. However, in order for this solution to remain effective when the throughput over the transmission channel diminishes and in order to avoid for example the visual quality of the viewed images from deteriorating for throughputs less than 1.4 bits per pixel, it is necessary to introduce an additional inter-image coding taking into account the natural temporal correlation of the image sequences, this additional inter-image coding possibly consisting in a differential coding with motion compensation at the level of each sub-band. Naturally, motion compensation is carried out at the level of the sub-bands and not at the level of the full-definition image (that is to say the non-split image) in such a way as to ensure compatible coding of the images and to avoid drifting at the level of the compatible decoders. Estimation of the motion takes place over the non-split images in accordance with a technique known as "block matching" in accordance with which the image is partitioned into blocks, one motion vector per block being determined through a correlation calculation. This technique enables motion vectors calculated as integer numbers of pixels to be obtained. An estimation of vectors to half-pixel accuracy can also be obtained by firstly interpolating the images for example with a bilinear filter. The motion vectors can thus be divided by the sub-sampling factor for each sub-band under consideration so as to serve as basis for the motion compensations to be carried out in each of the latter. Instead of extracting the coded pixel next to the preceding sub-band from the value of the pixel to be coded, the corresponding procedures consist in extracting the value of a neighbouring pixel whose coordinates are calculated as a function of these motion vectors. Thus, when the components of the motion vector are integers, the problem reduces solely to a memory addressing problem since it suffices to fetch an existing pixel from memory. On the other hand, if the motion vector does not correspond to an integer number of pixels, the value of the pixel sought can only be interpolated. However, the known methods of interpolation with a given band lead to the use of several long filters in order to take into account the information contained in the adjacent bands, thus posing embodiment problems which are difficult to solve.